Implantable cardioverter defibrillators (ICDs) can be beneficially used to automatically detect malignancies in patient cardiac function and cause cardiac function to return to normal or at least non-malignant function by delivering stimuli. Such abnormalities are often referred to as arrhythmias, and may include brady- or tachy-arrhythmias. Fibrillation is a tachyarrhythmia that may be atrial and/or ventricular in origin. Some arrhythmias such as ventricular fibrillation are considered malignant and warrant treatment by an ICD. There are many methods of detecting arrhythmias and determining whether an arrhythmia is malignant and therefore warrants treatment.
Once treatment is indicated, the goal is to convert a malignant arrhythmia to normal, or at least benign, cardiac rhythm by delivering one or more electrical stimuli, preferably in as short a time period as possible while conserving energy in response to these shockable conditions. An “episode” is a time period following identification of a shockable arrhythmia. During an episode, one or more stimuli may be delivered. An episode can terminate for several reasons including termination of the shockable condition (spontaneously or in response to stimulus) or identification of a timeout, such as when a predetermined maximum number of stimuli have been delivered. The present invention is aimed at improved and alternative devices and methods for delivering one or more stimuli in order to achieve successful conversion of malignant arrhythmias.